In general, a two-component developer used for an electrophotography is composed of non-magnetic toner and a magnetic carrier both of which are mixed together, and the carrier mentioned above gives necessary magnetism and appropriate electric charges to the toner through friction with the toner and it also has a function as a toner-carrier to attract toner electrostatically for conveying the toner.
As the carrier mentioned above, a resin-coated carrier wherein the surface of a magnetic core particle is covered with resin is valued highly from the viewpoint of durability and improvement in a property of triboelectrification.
As a method for forming a coating layer of the above-mentioned resin-coated carrier, there is available a wet method such as a method for spraying on a core particle a solution in which resin for coating is dissolved or dispersed or a method for soaking a core particle in the aforementioned solution. However, the wet method requires a long production time, provides poor yield and hardly offers coating layers in high quality.
With the background mentioned above, Japanese Patent Publication Open to Public Inspection No. 235965/1988 (hereinafter referred to as Japanese Patent O.P.I. Publication) discloses a technology for forming a resin-coated layer wherein the surface of a core particle composed of resin-powder-dispersed particles each having a weight average particle diameter of 20-200 .mu.m is coated with resin fine particles each having a particle diameter that is one tenth of that of the carrier or lower by means of a dry coating method, for obtaining a carrier for developing an electrostatic charge image.
On the other hand, Japanese Patent O.P.I. Publication No. 269544/1991 discloses a technology for forming a resin-coated layer wherein a coagulated substance having a volume base average particle diameter of 1.5-5.0 .mu.m composed of a plurality of primary fine particles each having a volume base average particle diameter of 0.5 .mu.m or less is stuck on the surface of a magnetic core particle through a dry coating method.
In the case of the technologies disclosed in the publications above, however, breakdown of a core particle caused by collision between core particles takes place when forming a coating layer by fixing resin particles on the surface of the core particle by means of a dry coating method, because only ranges of particle diameter of both a core particle and a resin fine particle for use in coating are watched for forming resin-coated carriers.
The aforesaid breakdown caused by collision between core particles results naturally in an occurrence of fine carrier particles which adhere to an image-forming unit and deteriorate image quality when they are used for image formation. Therefore, the aforesaid breakdown of a core particle is one of problems in the dry coating method for producing resin-coated carriers.
For solving the problem mentioned above, there have been proposed methods for easing the conditions on an apparatus including a method for lowering the peripheral speed for stirring in the course of dry coating. However, this has a problem that the layer-forming efficiency is lowered, or isolating resin fine particles stay and are contained in a carrier to adversely affect an image, and effective measures to solve the problem have not been found yet at this stage.
The inventors found, after their intensive study, that the problem mentioned above can be solved by the method wherein a range of particle diameter and a range of particle diameter distribution both for a core particle and a resin fine particle used for forming a coating layer on the surface of the core particle are regulated when forming resin-coated carriers through a dry coating method, and the core particles and the resin fine particles both having the regulated appropriate range of particle diameter and sharp distribution of particle diameter are used for forming carriers. Thus, the invention has been accomplished.
The present invention has been proposed based on the situation mentioned above, and an object of the invention is to provide a carrier for developing an electrostatic charge image wherein resin fine particles are fixed evenly and firmly to a magnetic core particle without any breakdown thereof through a dry coating method for the formation of a resin coating layer, and thereby no carrier fixes to an image-forming unit when forming an image and no white spot is caused and images with high image quality can be obtained stably for a long time.